Art. 128. LIVE LOAD FOR MAXIMUM CHORD STRESSES. 241 



truss (see Fig. 58, Art. 59), the vertical component of the diago- 

 nal in a panel corresponds with the vertical shearing stresses of 

 the solid beam; the horizontal component corresponds with the 

 horizontal shearing stresses; the increments of chord stress to- 

 ward the center, produced by the diagonals at the joints, corres- 

 pond with the increments of the bending stress; and the chord 

 stresses correspond with the resultants of the bending stresses. In 

 Fig. 58, section rs, l\=L 2 =33,3QQ Ibs. is the couple which forms 

 the moment of resistance. 



In a truss with inclined chords, the horizontal components 

 form the moment of resistance. In Fig. 177, the section 3-3, 

 through 0, (considering the part on the right) cuts off the forces 

 U 2 , T 2 , and L 3 . Considering the horizontal and vertical compo- 

 nents of U z , and taking center of moments at c, so that the mo- 

 ment of T as well as 

 of T 2 and L 3 shall be 

 zero, 



Hd=M c 



M c being the sum of 

 the moments of ex- 

 ternal forces about c. 

 But it is also true that 



L 3 d=M c =M c Fig. 177. 



It follows that H=L S and this is the couple which forms the 

 moment of resistance. 



Now since the horizontal componet of a chord stress is a 

 bending stress, and since the moment is always positive, the chord 

 stress itself must always be either compressive or tensile. It fol- 

 lows that the vertical component of a chord stress, which resists 

 a part of the shear, must always be of the same sign even though 

 the shear may reverse. 



129. Position of Live Load for Maximum Web Stresses. 



Let the shear in any panel of a truss be first considered. For 

 maximum shear in a panel, the same principle applies for deter- 

 mining the position of the live load as for determining the max- 

 imum shear at any cross section of a solid beam, but in a truss 

 both the positive and negative shears must be investigated (102). 

 The greater of these is usually called the positive shear, and the 

 lesser, the negative shear in the panel. 



